How DO you make diff only print out the lines that are common in more than one of two or more textfiles? I can’t figure it out. But I can help myself.

cat emails*.txt | sort | uniq -c | sort -n | while read num email ; do if [ $num -gt 1 ] ; then echo $email ; fi ; done

My name is Enno, and I have a bash problem. This is not my first time here at Bashers Anonymous.